The present invention relates to an electron gun for a color cathode ray tube, and more particularly, to an electron gun for a color cathode ray tube which can reduce haze around an electron beam spot generated when a vertical electron beam is over-focused by a nonhomogeneous magnetic field of a deflection yoke to thereby land on a fluorescent layer.
In general, an electron gun for a color cathode ray tube, mounted in the neck section of a cathode ray tube for emitting electron beams, comprises a cathode, a control electrode and a screen electrode, which form a pre-triode, a plurality of focus electrodes and anode electrodes for converging and accelerating an electron beam, in each of which three circular electron beam passing holes are formed.
Since the conventional electron gun for a color cathode ray tube constructed as described above converges and accelerates the electron beam generated from the cathode without changing its surface shape, when the electron beam is deflected to the periphery of a fluorescent layer under the influence of a deflection yoke, the electron beam is distorted by a nonhomogeneous magnetic field, thereby making it impossible to obtain a sharp picture.
To solve the aforementioned problem, as shown in FIG. 1, in the prior art, a recessed and horizontally elongated slot 12a is formed in the emitting surface of the screen electrode 12 located adjacent to the control electrode 11 of a triode to thereby correct for the influence of the nonhomogeneous magnetic field of the deflection yoke.
As described above, the formation of the horizontally elongated slot 12a in the emitting surface of the screen electrode 12 is for reducing the spherical aberration occurring within a main lens M and a deflection magnetic field and the perpendicular deflection distortion. However, it requires the prefocus lens P to be strengthened, thereby increasing the magnification of the pre-focus, causing the beam radius within the prefocus lens P to be reduced, thereby enlarging the virtual object point of a perpendicular beam, and finally resulting in the increase of the radius of the electron beam spot landing on a screen surface S. Therefore, although the deflection distortion in the center of the screen is reduced, since the radius of the electron beam spot therein is increased, the overall resolution is reduced, centering around the screen center which is a most vital part in the cathode ray tube. In other words, the image resolution in the periphery of the screen can be improved but the resolution in the center thereof is reduced.
To solve the aforementioned problem, as disclosed in U.S. Pat. No. 4,629,933 and shown in FIG. 3, in the prior art, an electron beam passing hole 21 of a control electrode 20 is formed as a horizontally elongated type and a vertically elongated slot 22 is formed in the emitting surface thereof, thereby forming and using horizontal and vertical crossover points differently. However, this may lower the focusing characteristics in the high current area. Also, since the slot 22 should be formed in the thin control electrode 20, the slot is manufactured such that a member wherein the slot 22 is formed is attached to the control electrode 20, or the control electrode 20 is compressing-processed. However, in case where the slot 22 is formed in the control electrode by welding the slot member thereto, the alignment which is a vital factor in the quality of an electron gun is deteriorated. Also, in case the control electrode 20 is compressing-processed, it is difficult to manufacture an electrode by means of molding.
Also, in U.S. Pat. No. 4,234,814 a conventional electron gun is disclosed in which a beam shape is elliptical at a main lens by making a crossover point of the electron beam emitted from a cathode different. The electron gun has a structure wherein a circular electron beam passing hole 25H is formed on a control electrode 25 and a horizontally elongated recess 26s is formed in the receiving surface of a screen electrode 26 formed opposingly thereto, as shown in FIGS. 4A and 4B.
The electron gun increases a vertical electron beam diverging power by the horizontally elongated recess 26s formed on the screen electrode 26 so that the crossover point of the vertical electron beam is positioned far from the cathode but the crossover point of the horizontal electron beam is positioned comparatively near the cathode.
In such a manner, positioning the crossover point of the vertical electron beam far from the cathode reduces the incident angle of the vertical electron beam with respect to the crossover point and reduces the emitting angle thereof from the crossover point, thereby making the diameter of the vertical electron beam in the main lens small.
Accordingly, the diameter of the vertical electron beam can be reduced in both the main lens and deflection field so that the electron beam haze due to a spherical aberration and deflection aberration is reduced. However, the object point radius cannot be reduced even by reducing the electron beam passing hole of the control electrode 25, which is one of the vital factors of determining focus characteristics. Thus, there is a limit in obtaining good focus characteristics.